This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-036690, filed Feb. 15, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for inspecting a projection exposure apparatus used for manufacturing a semiconductor device, and particularly to a method for inspecting performance of a projection optical system of an exposure apparatus.
2. Description of the Related Art
A lithography technique is generally used to manufacture a circuit pattern of a semiconductor device. In a projection exposure apparatus used in a lithography process, light emitted from an illumination optical system enters a photomask on which a circuit pattern is drawn. Light passing through the photomask is converged by a projection optical system. Further, in general cases, the circuit pattern of the photomask is focused and projected on a substrate applied with a photosensitive material, e.g., a silicon wafer applied with photoresist.
Recently, as the semiconductor device pattern to be formed is downsized, the dimension of the pattern to be formed by the optical lithography becomes severer.
In case of the exposure apparatus, as the pattern of the semiconductor device in comparison with the exposure wavelength is shrunk more, diffraction of light becomes more remarkable. Also, it is known that the diffraction angle increases as the period of the pattern decreases. To form a micro pattern, the diffraction light propagating in the direction in which it goes away from the optical axis needs to be captured and converged onto the wafer. Therefore, the diameter of the projection optical system needs to be increased in order to form a more microscopic pattern. In other words, the numerical aperture NA of a projection optical system needs to be increased. In case of the exposure using a photomask which has a one-dimensional periodic pattern such as line and space pattern, a plurality of discrete diffraction light are occurred. The discrete diffraction light are straight zeroth-order diffraction light, first-order up to higher-order diffraction light which have predetermined diffraction angle. In order to form a one-dimensional periodic pattern on the wafer, first-order diffraction light needs to be captured and converged with zeroth-order diffraction light.
Meanwhile, if the projection lens forming part of the projection optical system becomes large, a problem occurs in that light transmittance depending on the light path changes. In case of exposing a relatively large pattern with respect to the exposure wavelength, the light diffraction angle is small. In this case, only the portion of light that passes through the optical axis of the projection lens contributes to focusing of an image. That is, the paths of zeroth-order diffraction light and first-order diffraction light that are used for focusing an image are different from each other. Accordingly, the intensity of each diffraction light is not influenced by changes of the transmittance of the projection optical system.
In contrast, the diffraction angle is large in case of exposing a micro pattern, and therefore, zeroth-order diffraction light and first-order diffraction light are different from each other. Accordingly, if the transmittance in the projection optical system changes depending on light paths, diffraction light which reaches a wafer is influenced by changes of the transmittance, and as a result, the intensity of each diffraction light changes.
In conjunction of design of the projection optical system, changes of the transmittance depending on the light paths are not caused. But in practice, the drawbacks can be occurred imperfect anti-reflection coating on lens surface, light absorption of lens material, and the like. However, proposals have not yet been made for a method of directly measuring this phenomenon without disassembling the exposure apparatus.
A transmittance change depending on the light paths causes the intensities of zeroth-order diffraction light and first-order diffraction light to change. Since photoresist pattern on a wafer is formed by interference between these diffraction lights, a change of the intensities influences the pattern image focusing performance. As a result of this, it is considered that the micro pattern transferring performance of the projection optical system is deteriorated.
If a micro periodic pattern is formed by interference between zeroth-order diffraction light and first-order diffraction light, light generated by the interference constructs a bright part and a dark part. The degree of brightness is expressed as an amount of contrast. If bright and dark parts are clearly distinguished from each other, it is called xe2x80x9chigh contrastxe2x80x9d. The higher the contrast of interference light, the easier the transfer of the pattern onto the wafer. In other words, the contrast should desirably be high in order to widen the focus margin and the exposure dose margin. The contrast is determined by amplitude and phases of lights which interference each other.
If a circuit pattern is designed supposing that drawbacks described above do not occur, the contrast of interference light formed on the wafer is rendered insufficiently high. As a result, no pattern may be formed. At present, shrinkage of patterns has progressed and lithography design using simulations has come to have a significant meaning. It is undesirable that unexpected drawbacks of this kind occur in the exposure apparatus. In the process of assembling an exposure apparatus, drawbacks should be removed or extents of drawbacks should previously measured and which then have to be taken into consideration in case of estimate and designing of to-be-formed pattern based on exposure simulations.
An example of measurement of contrast, which has been conventionally carried out, will be explained with reference to FIG. 1. FIG. 1 shows relationship between formed photoresist patterns (left side) and relative light intensities I (=1/D) (right side). The contrast is expressed by the following expression with use of a light intensity I1 at peaks of light intensity and a value I5 at a minimum light intensity between peaks.                               (          contrast          )                =                              (                          I1              -              I5                        )                    /                      (                          I1              +              I5                        )                                                  =                              (                                          1                /                D1                            -                              1                /                D5                                      )                    /                      (                                          1                /                D1                            +                              1                /                D5                                      )                                                  =                              (                          D5              -              D1                        )                    /                      (                          D5              +              D1                        )                              
In the expression, the intensity I5 at which the light intensity comes to peaks is an intensity of the minimum between peaks of light intensities. Although presence or absence of reduction of the contrast can be confirmed by the method shown in FIG. 1, factors which cause reduction of the contrast is very difficult to specify.
Another phenomenon which is caused by a change of the diffraction intensity is a positional shift of pattern depending on focusing onto a wafer, which is caused by the gravity center of the intensity of the diffraction light shifts from the center of the projection optical system. Where a line-and-space pattern are cited as an example, two of positive and negative first-order diffraction lights are generated with a center of zeroth-order diffraction light taken as a symmetry point. If there is a difference between intensities of the positive and negative first-order diffraction lights, the position where the pattern is formed shifts depending on the defocus amount of the wafer.
The shift of the position where the pattern is formed depending on the defocus amount of the wafer is occurred due to factors other than a transmittance change depending on the light paths, such as coma aberration or illumination telecentricity error. Therefore, it is difficult to specify the factor which causes the shift of the position only by the measurement of the relationship between the defocus and misalignment of the pattern.
The present invention has an object of providing a method for inspecting an exposure apparatus capable of specifying a change of the light transmittance depending on the light path.
The present invention provides a method for inspecting an exposure apparatus, comprising: a step of guiding light emitted from an illumination optical system to a photomask where a pattern is formed of an optical member including a light transmission pattern as a diffraction grating pattern, in which a light transmission part and a opaque part are repeated in a predetermined direction, a plurality of ratios are given between a length of the light transmission part and a length of the opaque part in a repetition direction, and a periphery of the light transmission pattern is shielded by a opaque area, such that a plurality of ratios are given between the light transmission part and the opaque part;
a step of irradiating diffraction light, which has passed through the photomask, on a projection optical system, thereby to transfer a pattern reflecting an intensity distribution of the diffraction light to a wafer; and
a step of measuring a change of transmittance depending on a light path of the projection optical system, based on a pattern image of the diffraction light transferred to the wafer.
When inspecting a projection optical system of an exposure apparatus in the present invention, light emitted from a light source is guided to a photomask and light passing through the photomask is irradiated on like normal pattern exposure, thereby to transfer a pattern image reflecting an intensity distribution of the diffraction light on a wafer.
When transferring a pattern in the present invention, a light transmission pattern in which light transmission parts and opaque parts are repeated at a finite period is formed on a photomask, and therefore, diffraction light is obtained.
In addition, the photomask and the wafer are rendered non-conjugate with respect to the projection optical system. In this manner, pattern transfer can be performed in a state in which diffraction lights of zeroth-order up to higher-order are separated from each other and diffraction light components have sufficient sizes. In the present invention, where NA is a numerical aperture of the projection optical system in a wafer side, xcex is a exposure length, "sgr" is a coherence factor, and M is a magnification of the photomask, the period of the diffraction grating pattern is set so as to satisfy p greater than Mxcex/NA(1+"sgr"). In this manner, first-order diffraction light can be transferred to the wafer without being shaded by the aperture stop, so that the light intensity distribution can be inspected based on the transferred pattern image.
By observing patterns thus obtained on a wafer, it is possible to measure changes of light transmittance depending on light paths.
Specifically, by taking exposure using light transmission patterns which have the light transmission parts and the opaque parts the ratio of which (the ratio between the light transmission parts and the opaque parts) are different from each other, a plurality of resist patterns are transferred onto the wafer. By analyzing the resist patterns, an equal-intensity contour plot of light intensity distributions is obtained. It is thus possible to measure the light transmittance depending on the paths of the projection optical system based on the contour plot of light intensity distributions.
More desirably, the photomask where the diffraction patterns are formed is constructed as a attenuated phase shift mask. Namely, the diffraction pattern is constructed by a light transmission part and a semi-transparent phase shift part at which the phase is shifted from the light transmission part. In this respect, the duty ratio of the diffraction grating can be adjusted so that the intensity of zeroth-order diffraction light can be approximately zero. In this case, patterns depending on the zeroth-order diffraction light are not transferred. Therefore, only the first-order diffraction light can be observed so that first-order diffraction light components close to the light axis of the optical system of the exposure apparatus can be observed.
In addition, the non-conjugate state in which the photomask and the wafer are non-conjugate with respect to the projection optical system is realized by arranging the opaque part of the light optical member on a surface opposite to a surface where the optical member of the photomask used for device pattern exposure is arranged. That is, the photomask is attached to a mask stage, reversed from the state in case of device pattern exposure. In this manner, a non-conjugate state can be created very simply while maintaining the structure of the exposure apparatus used for pattern exposure. Of course, at least one of the photomask and the wafer may be shifted from a conjugate position in the light axis direction.
Also, conditions are set so as to satisfy a relationship of 0.4(ndxcex)1/2xe2x89xa6rxe2x89xa6(ndxcex)1/2 where the light transmission pattern has a circular shape having a radius r, d is thickness of the photomask, xcex is an exposure wavelength, and n is a refractive index of material of the photomask at the exposure wavelength xcex. In this manner, the resolution of the transferred pattern image can be improved and the resist patterns which are suitable for measurement can be obtained.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.